More trees will not compensate for growing carbon emissions
the existing belief that planting trees to absorb carbon dioxide ( co 2 ) is an alternative to reducing fossil fuel emissions is slowly losing ground. Now, scientists are predicting that carbon sinks -- forests -- will soon become saturated with co 2 and begin returning a major part of it to the atmosphere, thus temporarily accelerating global warming.
Over six billion tonnes of co 2 is released into the atmosphere each year and about one-third is absorbed by the world's forests. This led to the belief that reduction in co 2 concentration is possible if tree cover is increased. In the process, scientists overlooked another possibility -- that trees also release co 2 while breaking down the sugar produced during photosynthesis, a process called respiration.
While low levels of co 2 limit photosynthesis, higher levels promote forest growth and the accumulation of carbon in forest soils, a phenomenon called co 2 fertilisation. As forests grow, they absorb more co 2 , helping to stave off climate change. But that is so just for a while. Until a few months ago, researchers had assumed that as long as co 2 levels were rising, the carbon sink would continue to grow. The un Intergovernmental Panel on Climate Change's (ipcc) latest assessment, published in 1996, concluded that forests would soak up around 290 billion tonnes of carbon in the next century even without extra planting. This now seems highly unlikely. Experts such as Bob Scholes, who works with a research agency funded by the South African government, argue that co 2 fertilisation has already peaked and respiration may accelerate. Thus, he adds, forests further planted to protect the world from global warming could be doing the opposite.
Scholes explains that this confusion was caused by a time lag. co 2 fertilisation is an instantaneous process but respiration increases in response to temperature rise triggered by co 2 . This warming has a built-in delay of about fifty years, caused largely by the thermal inertia of the oceans. So, the extra outpouring of co 2 from the world's forests would not yet be apparent. "During this delay, there is an apparent carbon sink," he says.
But notwithstanding all these findings, the tree planting drive should not be stopped as they will always be keeping some carbon dioxide out of the atmosphere and provide other ecological benefits. Measures to control fossil fuel emissions should be taken on a war footing. "The carbon cycle has a very long equilibrium time," says Scholes. The consequences of decisions taken now will persist for many centuries, he adds.
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